if you are not getting progress updates it could be related to buffering of script output on server side, this can be affected by different server settings.

Please also note that sitemap generator can be used for own personal sites only , i.e. it cannot be used to create sitemaps for 3rd party sites or provided as a service (paid or free): http://www.xml-sitemaps.com/license.html (separate licenses need to be purchased for that)